The present invention relates to nuclear radiation detectors, and more particularly to self-powered detectors and their method of manufacture.
A self-powered nuclear radiation detector is one in which no drive potential need be applied to the detector to produce a signal as a function of the neutron or gamma radiation flux incident on the detector. The output signal from a self-powered detector is produced as a result of the differing radiation interaction characteristics of the spaced apart, insulated emitter and collector electrodes. Such a detector is taught in U.S. Pat. No. 4,080,533, and includes a centralized emitter electrode and a coaxial collector electrode about the emitter and insulated therefrom by a refractory insulating oxide.
The accuracy of the output signal as a measure of the radiation flux level is highly dependent upon the emitter diameter and length, with the emitter diameter desired to be uniform along its length and the length to be accurately determined. A detector is remotely positioned from the signal indicator or meter which is typically outside the radiation flux field. A signal cable must be connected to the detector. The signal cable is typically a cable with the center wire aligned with and electrically connected to the detector emitter.
It has been the practice to fabricate the detector to close dimensional tolerances separately, and to then join this detector of desired length to the signal cable as by brazing together the emitter and cable center wire, and brazing the outer sheaths. The junction between the detector and the signal cable has been difficult to make reliably, due to the small diameters of the fabricated devices. The junction was easily broken during testing and handling, or during insertion of the detector into the reactor.
Another fabrication technique practiced in the prior art had been to join the emitter wire and signal cable center wire before reducing the diameters of the emitter and the cable center wire to their desired values. The joined emitter wire and cable center wire were then placed within a continuous tubular sheath with insulation compacted about the emitter and center wire, and thereafter the entire assembly swage-reduced to a desired diameter. This technique tended to compromise on either the final emitter wire diameter or length with resulting variations in detector sensitivity.
It is important to produce detector and cable assemblies which has accurate, reproducible sensitivities. This is because the signal levels being generated are typically very small, in the nanoamp range. Also, the detectors have a finite operating life due to changes in emitter radiation response over time. This means that the detector and cable assemblies will be periodically replaced, and without repeatable radiation response from assembly to assembly it is difficult to interpret the reactor operation conditions.
It is an object of the present invention to be able to fabricate a detector and signal cable assembly in which the detector emitter diameter and length are very accurately determined, to permit good quality control of the fabricated assemblies radiation sensitivity.
It is also an object to produce detector and signal cable assemblies with a strong reliable junction between the emitter wire of the detector and the signal cable center wire.
In U.S. Pat. No. 4,087,693, a self-powered detector is fabricated by welding together the ends of a rhodium emitter wire and a signal cable wire. An insulating sheath of woven silicon dioxide fiber is placed over the joined emitter and signal cable wire. A tubular, collector electrode is then placed over the insulating sheath and is slightly reduced in size to compact the insulating fiber sheath and center the emitter wire. There is no significant change in the emitter wire diameter. An end plug is then provided at the extending end of the emitter, with insulation between the emitter end and the plug.
It is desirable to be able to fabricate long emitter wire detectors, which are more practically made by starting with a larger diameter emitter and swage reducing the emitter wire during fabrication. This swage reducing or uniform elongation with reduction in diameter is a convenient way of providing uniform diameter elongated detectors.